Vehicle number plates (i.e. license plates) are commonly made to be retroreflective in order to enhance the visibility of the number plate at night.
Certain methods of fabricating retroreflective number plates commonly involve the lamination of a thin retroreflective film (also commonly referred to as a retroreflective sheeting) to the back-side of a thick, clear plastic plate. The lamination of the retroreflective film to the clear plastic plate is often achieved with an optically clear pressure-sensitive adhesive. The clear plastic plate is commonly made from either acrylic or polycarbonate resin and is typically about 0.125 inches thick.
When retroreflective number plates are used for the purposes of vehicle registration, the number plates applied to each individual vehicle must contain a unique set of alphanumeric characters or other characters or symbols. These characters are printed (or otherwise created) on either the back surface of the plastic plate or on the top surface of the retroreflective film prior to laminating the retroreflective film to the plastic plate. The most common method currently employed to print these characters is to use a computer-controlled printer such as a thermal transfer printer, an ink jet printer, or a laser printer to print the characters on the front-surface of the retroreflective sheeting.
The retroreflective film employed to fabricate a number plate can include any of commonly known retroreflective sheeting constructions and can incorporate any of the commonly known retroreflective elements including glass microspheres or microprisms. The three most common retroreflective sheeting constructions include enclosed lens retroreflective sheeting (incorporating the glass microspheres), encapsulated lens retroreflective sheeting (also incorporating glass microspheres) and microprismatic retroreflective sheeting. U.S. Pat. Nos. 2,407,680 and 4,367,920 provide detailed descriptions of the design and manufacture of enclosed lens sheeting and are incorporated herein by reference. Also, U.S. Pat. Nos. 3,109,178 and 4,025,159 provide detailed descriptions of encapsulated lens sheeting, and U.S. Pat. Nos. 3,689,346 and 4,588,258 provide descriptions of microprismatic sheeting, which are incorporated herein by reference.
Not all retroreflective sheeting materials, however, can be successfully printed through thermal transfer printers or other printers. To be acceptable for use in the fabrication of numbered plates, a high print quality is required. Generally, the print quality is judged across several different criteria. First, the coverage of the print must be complete without any signs of pin-holes or other print voids, and the depth of coverage should be full enough to provide a deep, fully saturated print. When printing opaque colors such as black, the surface of the retroreflective sheeting should not show through the print. Second, the edges of printed alphanumeric characters must be straight, clean and sharp. Corners should be square. Print edges that are wavy or undefined are generally not acceptable. Third, the printed surface should be uniform without any smudging or smearing. This smearing or smudging effect is especially common with thermal transfer printing when printing letters such as “E”, “F”, “H”, or “T” from left-to-right where there exists a long horizontal bar of printing.
Certain conventional methods have attempted to overcome these printing challenges by creating a matte surface on the face of the enclosed lens sheeting during conventional manufacturing processes. Though these conventional approaches have allowed for an improved print quality, they do not always provide consistent print results, and there is a significant potential for scrap using this method of manufacturing. Moreover, the most pronounced problem with this approach is the presence of pin-holes or printing voids when thermal transfer printing on the surface of the enclosed lens sheeting. These defects are often caused by surface imperfections in the top-coat of the retroreflective film, including air-bubbles, surface pits, pin-holes, and similar coating defects.